If the Fateless One decides to work with Corialia then Matrim Hawkins must be killed to remove any evidence of Corialia's practices.

Matrim is a neutral NPC and therefore must be assassinated. Care must be taken to do it without witnesses. However, if you speak to him first, he will know that you have made a deal with Corialia & fight you. Speaking to him first will remove any chance of being caught for murder.

Once that's done the Fateless One should return to Corialia (level-based XP ~3450 at level 37 and a random necklace, but no Gold).